The control systems generally used comprise control loops, optionally in cascades, to make it possible to regulate an intermediate quantity on which the influence of the perturbations shows itself more rapidly than on the principal quantity controlled.
It is well known that positioning systems having several loops in cascades can be used. FIG. 1 shows a device of the prior art making it possible to regulate the movement of a strip 10 wound on a roll 11 driven by a motor 12. The motor 12 drives the roll 11 in the direction of the arrow 13 so as to reel out the strip 10 at a speed dy/dt. The strip 10 supplies a machine (not shown) at a feed speed dx/dt. So as to be able to control the speed of rotation of the motor 12 as a function of the feed speed dx/dt of the strip into the machine (not shown), it is necessary to measure the variation in speed between the feed into the machine and the speed at which the strip is reeled out. In order to do this, three idler rollers 14, 15, 16 are used, enabling the direction of the strip to be changed. The roller 16 is drawn back by a spring 17, a weight, a jack, or any other similar component. The position of the roller 16 is a function of the difference between the feed speed dx/dt and the reeling-out speed dy/dt. Normally the speed of rotation .OMEGA. of the motor 12 is measured by means of a tachometer 18. The speed at which the reel should rotate is derived from the position of the roller 16. It is compared at 19 with the signals coming from the tachometer so as to derive from this an error signal which is sent to the motor 12.
Advantageously, PID devices with proportional, integral and derivative action well known in the art are used, which make it possible to obtain actions giving good stability whilst diminishing the transitory phases. However, the setting of the parameters of these PID devices is long and tedious since it is carried out by successive approximations and the actions on one of the parameters generally entail modification of the other parameters. These control chains are generally of analog type. However, the arrival on the market of digital PID devices, made possible by the current stage of development of electronics, now allows digital processing by means of analogue/digital converters.